With all of the media coverage going on right now about the disaster in Japan, perhaps a bit of explanation is in order. (Warning for those of you versed in the world of nuclear physics: this is going to be a relatively simple, watered-down, and incomplete idea of what goes on in a nuclear reactor...don't get mad at me!) And let me get something out of the way right from the get-go: there's not going to be a nuclear explosion in Fukushima, Japan. While atomic bombs and nuclear power plants both rely on nuclear reactions, they are extremely different when it comes to their potential to explode.
So here's the short version. Essentially, nuclear reactors work in the exact same way as certain other engines we've been using for hundreds of years: by using steam. At the heart of a nuclear reactor lies a chamber that is submerged in large tank of water. Inside this chamber are a number of uranium "cores," if you will. These are about the size of a Tootsie Roll, and they're totally awesome.
Why are they awesome? When the reactor is in operation, each of these cores undergoes a self-propagating nuclear reaction in which uranium atoms split apart and release massive amounts of energy. Within the chamber are millions of neutrons floating around, causing this reaction to occur. The result of all this energy is a TON of heat, and since the reactor is submerged in water, this heat creates steam that leaves the chamber and powers engines. Think of the nuclear reactor as a kind of atomic furnace, burning uranium instead of wood.
So what happens when this all breaks down? You may have noticed that I said this was a self-propagating process. That means that you can't just stop it at will. As long as there are neutrons in the uranium chamber, nuclear fission will occur. Luckily, there are two general mechanisms to control this process. One is the water that surrounds this uranium chamber. The steam that is generated by this process is constantly being collected, cooled, and then recirculated through the system. This means that (when things are working properly), there is always a pool of water to capture the heat and keep things relatively stable.
The second control mechanism is a series of rods that can be inserted into the uranium chamber. These can be made of a number of different materials, but their key function is to "soak up" the neutrons in the chamber. As you'll recall, these neutrons are what drives the nuclear reaction, and removing them will cause this process to slow down (something you want in the event of catastrophe). If you want to slow the reactor as much as possible, you insert every rod into the core entirely and at the same time. This is called scramming the reactor, and it doesn't stop the reactor; it only slows it down.
What happened at the Fukushima plant? Believe it or not, the reactor held up to the earthquake just fine. All of the proper safety protocols were executed, and things seemed to be OK. However, safety engineers weren't prepared for a giant tsunami, and when key components of the power plant's backup electricity system were submerged in water, the reactor stopped recycling the steam that was created. As a result, two bad things happened: one was the gradual loss of the water surrounding the uranium core (since it was being turned to steam without a replacement), and the other was the increasing buildup of the steam itself.
The first is a problem because that water served as a way to carry the heat away from the uranium fuel. Without that extra cooling, the cores become so hot that they can melt. This is what is known as a "nuclear meltdown." The second (and more dangerous) problem of steam buildup arises from an interesting property of water. When steam (aka vaporized water) is heated to extremely high temperatures, it can actually cause the water molecules to break down into their component parts, hydrogen and oxygen. The important factor here is the hydrogen gas, an incredibly flammable substance that has a tendency to cause explosions. Given the super-hot temperature of the reactor, the hydrogen can explode, launching radioactive materials into the environment. In addition, if a fire is started, then smoke from the fire can carry even more radioactive particles into the air and beyond (a la Chernobyl).
We aren't sure yet what exactly happened at Fukushima, but rest assured it was not as bad as the media would have you believe. While there is a certain amount of radiation emanating from the power plant, it is far below the amount that is dangerous to humans. The situation at Fukushima could still change in a bad way, but as of now it seems that the damage has been contained. As we continue monitoring the situation in Japan, remember that this could have been a much worse catastrophe, had the proper safety mechanisms not been in place. All things considered, nuclear technology is a surprisingly safe way to get energy without carbon pollution. In the coming weeks there will be a lot of sensationalist talk about what happened at Fukushima. In light of such information, I urge you make an effort to separate fact from fiction, and as the great Douglas Adams would suggest, "Don't Panic!"
If you're still confused or curious, check out Stuff You Should Know's recent podcast on the disaster.